1. Field of the Invention
The present invention relates generally to a charge-coupled imager of inter-line transfer type and more particularly to a charge-coupled imager of inter-line transfer type suitable for use with a video processing apparatus such as a machine-vision in which an electronic shutter is controlled by varying an effective charge accumulating time of a charge-coupled device (CCD) without man's visual sense and so on.
2. Description of the Prior Art
Japanese Patent Application No. 61-252375 describes a solid-state imager in which an exposure time can be electrically controlled without a mechanical shutter. In such electronic shutter for a solid-state imager, the electronic shutter is driven by varying a time in which a charge is accumulated in a sensor or charge accumulating region of a charge-coupled device. The charge accumulated in the charge accumulating region is transferred therefrom to a vertical register in response to a sensor gate pulse. In other words, a duration in which the supply of a shutter pulse is inhibited and a next sensor gate pulse is applied is represented as an exposure time of shutter, and this exposure time can be varied.
FIGS. 1A to 1E are timing charts to which reference will be made in explaining an operation of the above-described electronic shutter. A sensor gate pulse shown in FIG. 1C is generated in synchronism with a vertical synchronizing signal VD of FIG. 1A. A charge accumulated is transferred from a sensor to a vertical register in response to the sensor pulse supplied. Shutter pulses shown in FIGS. 1D and 1E are supplied to the charge-coupled imager in response to a horizontal synchronizing signal HD of FIG. 1B. When the shutter pulse is supplied, the charge accumulated in the charge accumulation region is delivered to an overflow-drain (OFD) of the charge-coupled device. The shutter pulse is not generated in the charge accumulating time so that a time period in which the generation of the shutter pulse is inhibited is longer in the shutter pulse of 1/500 second than that of the shutter pulse of 1/1000 second. Examining the shutter speeds of 1/500 second and 1/1000 second with reference to FIG. 1, it is to be understood that the ending time of the charge accumulating time is constant and that the shutter speed is varied by changing the timing of the charge accumulating time.
According to the prior-art charge-coupled imager, the ending time of the charge accumulating time is fixed and the starting time of the charge accumulating time is varied. There is then presented such a problem that, when a cameraman takes a picture in a high-speed mode by the use of a so-called machine vision or the like, the cameraman can not capture an object while the charge accumulation is started. Further, in a still camera or the like, a shutter speed is determined by varying the ending time of the exposure time. From this shutter standpoint, it is natural that while the charge accumulating time is determined, the ending time of the charge accumulating time is varied, which is particularly desired by a video camera apparatus such as a machine-vision or the like.
Furthermore, it is proposed to vary the ending time of the charge accumulating time by utilizing a charge-coupled imager of a so-called frame inter-line transfer-type which is the combination of the inter-line transfer type and the frame-transfer type. This previously-proposed technique costs very much.